The present invention relates to a process for the elevated temperature preparation of copolymers comprising monomers of methacrylic acid, conjugated diolefins and esters of .alpha.,.beta.-ethylenically unsaturated dicarboxylic acids which serve as viscosity-raising agents.
Viscosity-raising agents (thickeners) are used, inter alia, as auxiliary agents in coating compositions for the paper and textile industry. Natural as well as synthetic compounds are employed in such agents. The synthetic products are of special interest industrially since they can be obtained in uniform quality.
One group of such synthetic products is constituted by dispersions of copolymers produced from a conjugated diolefin and methacrylic acid. Optionally, esters of .alpha.,.beta.-unsaturated dicarboxylic acids can also be included. Thus for example, DE-OS (German Unexamined Laid-Open Application) No. 2,035,919=GB-PS No. 1,271,282 discloses dispersions of copolymers of butadiene and an unsaturated carboxylic acid as thickeners. A disadvantage connected with these products is that the polymerization of the dispersions described in the reference can be controlled only with great difficulty due to the vigorous evolution of heat.
Furthermore, other thickeners containing dispersions are disclosed in DE-OS No. 2,025,696=U.S. Pat. Nos. 3,657,175 and 2,250,977=U.S. Pat. No. 3,903,035.
These are composed of at least terpolymers of butadiene, an unsaturated carboxylic acid, as well as half esters of .alpha.,.beta.-unsaturated dicarboxylic acids. An essential disadvantage of these products is the requirement that the ester monomers must be separately produced, since they are not commercially available. As a result, these thickeners are unacceptably expensive.